Signal rectifier circuits are commonly used for level sensors responsive to an input signal's level in prior art electric or electronic equipment. For example, noise reduction circuits or automatic recording level control circuits in audio or video tape players use this sort of signal rectifier circuit.
Signal rectifier circuits operate to rectify an AC input signal, for example, an audio or video frequency signal, to a D.C. signal. As is well-known, ripple components of the D.C. signal are reduced by passing the D.C. signal through a smoothing circuit. The time constant of the smoothing circuit is automatically shortened when a high level signal is suddenly inputted, so that the attack time or the compliance of the circuit to the high level signal is improved.
In a conventional signal rectifier circuit, for example, the control circuit in FIG. 3 of U.S. Pat. No. 3,775,705, the smoothing circuit has a first RC smoothing network formed by a resistor and a capacitor with a relatively short time constant. A second network comprising a series resistor and a shunt capacitor follows the first network and a diode with some forward voltage drop is connected in parallel with the series resistor. The series resistor in the second network is shunted by the diode when, for large increases in signal amplitude, the diode conducts. The time constant of the smoothing circuit is thereby reduced. However, such a conventional rectifier circuit is defective in that the threshold level of the input signal necessary for the time constant change is fixed to the forward voltage drop of the diode, for example, with a silicon diode about 0.7 volts. In other words, it is very difficult to select a different threshold level. Moreover, formation of two capacitors in such a prior art circuit is difficult when the signal rectifier circuit is manufactured in the form of an integrated circuit.